(1) Field of the Invention
The present invention relates to a method for forming negative and positive images in the electrophotographic process. More particularly, the invention relates to an electrophotographic process in which either negative images or positive images can be formed only by electric controls by using an ordinary electrophotographic light-sensitive layer and a single one-component type magnetic developer.
(2) Description of the Prior Art
In the field of the electrophotography, there is a strong desire for a multiple-objective copying system in which not only ordinary reproduction for forming a positive copy from a positive original but also formation of a reversed positive image from a negative original such as a microfilm or from a negative electrostatic latent image such as a laser beam or light emitting diode array is possible.
Conventional negative/positive reproduction systems are roughly divided into two types. According to one type of the conventional method, a bichargeable photosensitive material and a toner having a tendency to be frictionally charged with a certain polarity are used, when a positive image is obtained, the polarity of the charge of the photosensitive layer is made opposite to the charge polarity of the toner, and when a negative image is formed, the polarity of the photosensitive material is made the same as that of the toner and a reversed image is formed. However, the number of bichargeable photosensitive materials is considerably limited and hence, the durability and sensitivity are restricted. Furthermore, the sensitivity and other electrophotographic characteristics of the photosensitive material greatly differ between the case of positive charging and the case of negative charging, and it is very difficult to adjust the densities and qualities of negative and positive images to substantially equal levels.
According to another type of the conventional method, a photosensitive material which is chargeable with a certain polarity is used, an electrostatic latent image is formed by charging and imagewise light exposure, when a positive image is formed, development is carried out by using a toner charged with a polarity reverse to the polarity of the electrostatic latent image, and when a negative image is formed, reversal development is carried out by using a toner having the same polarity as that of the electrostatic latent image. This method is defective in that two kinds of toners should be prepared and the toner-exchanging operation is troublesome.